Image Stretching as result of 3d tracking
The most important piece of information is at the top left of your image: you are using the movie distortion node to unidistort the video (I presume, since you didn't bother showing the complete node tree). If you undistort a pinchushioned distorted image, the resulting image will  no longer fill the edges of the frame.
You have two options, use the original distorted image (so that it fills the frame like the original one) and distort the 3D elements so that the integration looks correct.
Or you can live with the distortion-corrected image and the undistorted 3D elements and re-scale the composition at the end, but you will lose some pixels in the edges.
Please read the following link for more info:
How to determine lens undistortion values for motion tracking?